Phase-change optical recording medium and recording method and apparatus for the same

ABSTRACT

An optical recording medium has a phase-change recording layer containing Sb and Te as essential elements therefor, to which is added at least one element selected from the group consisting of Ag, Au, Cu, Zn, B, Al, Ga, In, Si, Ge, Sn, Pb, N, P, Bi, La, Ce, Gd, and Tb, the recording layer being capable of assuming an amorphous phase changed from a crystalline phase by the application of a laser beam thereto, thereby optically recording information. Recording marks are formed in the recording medium by converting a light emission wave of laser beam into a recording pulse train comprising a plurality of on-pulses and off-pulses, with a recording frequency being continuously changed corresponding to the location of each of the recording marks in the radial direction of said recording medium. A recording apparatus has laser beam driving circuit means, signal generation means, and signal transmission means for achieving the above recording method.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of applicants' copending ContinuedProsecution Application Ser. No. 09/568,723 filed May 11, 2000 now U.S.Pat. No. 6,548,137.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical recording medium comprisinga phase-change recording material which is optically changeable by theapplication thereto of a light beam so as to carry out recording,reproducing, and overwriting of information, and more particularly to aphase-change optical recording medium capable of recording informationwith high density at high recording linear velocity.

In addition, the present invention also relates to a recording methodand apparatus for the above-mentioned phase-change optical recordingmedium by a Constant Angular Velocity (CAV) system or a Constant LinearVelocity (CLV) system which is achieved by dividing the recording areaof the recording medium into a plurality of sections in the radialdirection of the recording medium.

2. Discussion of Background

To reproduce or record information in an optical disc with high densityby using a laser beam, there is conventionally known the overwritingmode in which recording marks are formed in the optical disc by theapplication of a plurality of short-length pulse trains as disclosed inJapanese Laid-Open Patent Application 3-185628. However, theabove-mentioned overwriting mode has the drawback that the laser powerbecomes partially insufficient. More specifically, in the case where theoptical disc is rotated at a constant number of revolutions, therelative speed of the laser spot is not constant between a recordingarea at the inner circumference of the disc and that at the outercircumference thereof, viewed in a radial direction of the disc. In sucha case, the laser power becomes insufficient in the area where therelative speed of the laser spot is fast. Furthermore, there will occura new problem that design of the circuit becomes difficult.

To solve the above-mentioned problems, there is a proposal in JapaneseLaid-Open Patent Application 6-12674. This proposal is that when aninput signal with a particular wave form, for example, an eight tofourteen modulation signal (EFM signal) is given to an optical disc, thewave form of laser is modulated depending upon the linear velocity. Morespecifically, the laser is modulated into a short-length train of pulseswhen the linear velocity is slower than a predetermined value (L0); andthe laser is modulated into one pulse which is made slightly shorterthan the corresponding recording mark when the linear velocity is fasterthan the aforementioned value (L0).

There is an increasing demand for development of a phase-change opticalrecording medium and a recording method therefor, which will be able toachieve high-density recording such that the recording capacity thereofis the same or more than that of DVD-ROM, and attain high-speedrecording at a linear velocity of 2 times or more (about 7 m/s or more)that of the nominal speed for the DVD-ROM. However, when such an opticalrecording medium is subjected to the above-mentioned CAV or CLVrecording method, it is conventionally known that good recordingcharacteristics cannot be obtained with respect to jitter value byslightly shortening the input pulse width. On the contrary, when theaforementioned conventional method, as proposed in Japanese Laid-OpenPatent Application 6-12674, of modulating laser into a short-lengthtrain of pulses at the lower linear velocity side, good results can beproduced. It is considered that, in Japanese Laid-Open PatentApplication 6-12674, the above-mentioned recording method is employedbecause the optical disc employs a composition close to a compound ofGe₂Sb₂Te₅.

However, when recording is carried out using the modulated laser withthe pulse width being fixed so as not to be deformed at the higherlinear velocity side, the pulse width becomes too short in the recordingarea of the lower linear velocity side, that is, at an innercircumference of the recording medium. In this area, there is a tendencyof impairing the jitter value due to insufficient recording power. It isconsidered that such a phenomenon is also caused by the composition of arecording layer for use in the optical recording medium.

SUMMARY OF THE INVENTION

Accordingly, it is a first object of the present invention to provide aphase-change optical recording medium free from the above-mentionedconventional drawbacks, capable of achieving high-density recording suchthat the recording capacity thereof is the same or more than that of theDVD-ROM, and attaining high-speed recording at a recording linearvelocity in the range of 3.0 to 20 m/s.

A second object of the present invention is to provide a recordingmethod for the above-mentioned phase-change optical recording medium.

A third object of the present invention is to provide a recordingapparatus for the above-mentioned phase-change optical recording medium.

The above-mentioned first object of the present invention can beachieved by an optical recording medium for recording information,comprising a phase-change recording layer comprising Sb and Te asessential elements therefor, to which is added at least one elementselected from the group consisting of Ag, Au, Cu, Zn, B, Al, Ga, In, Si,Ge, Sn, Pb, N, P, Bi, La, Ce, Gd, and Tb, the recording layer beingcapable of assuming an amorphous phase changed from a crystalline phaseby the application of a laser beam thereto, thereby optically recordinginformation.

In the above-mentioned optical recording medium, the Sb and Te, and atleast one element selected from the aforementioned group constitute aeutectic phase-change material, with Sb and Te serving as the maincomponents therefor, and at least one element serving as an additionalcomponent in an atomic percentage of 17% or less in the eutecticphase-change material.

It is preferable that there is a difference in reflectance of 30% ormore between (a) the crystalline phase and (b) the amorphous phaseformed by the application of the laser beam to the crystalline phase ata recording linear velocity ranging from 3 to 20 m/s.

The second object of the present invention can be achieved by a methodfor optically recording information, using an optical recording mediumcomprising a phase-change recording layer comprising Sb and Te asessential elements therefor, to which is added at least one elementselected from the group consisting of Ag, Au, Cu, Zn, B, Al, Ga, In, Si,Ge, Sn, Pb, N, P, Bi, La, Ce, Gd, and Tb, the recording layer beingcapable of assuming an amorphous phase changed from a crystalline phaseby the application of a laser beam thereto, thereby optically recordinginformation by forming recording marks therein, wherein when therecording marks are formed in the optical recording medium, a lightemission wave of the laser beam is converted into a recording pulsetrain comprising a plurality of on-pulses and each off-pulse subsequentto the on-pulses, with a recording frequency ν (ν=1/Tw where Tw is awindow width) being continuously changed corresponding to the locationof each of the recording marks in the radial direction of the recordingmedium, either in the direction from an inner circumference towards anouter circumference of the recording medium, or in the direction fromthe outer circumference towards the inner circumference of the recordingmedium.

The third object of the present invention can be achieved by a recordingapparatus for recording information comprising laser beam drivingcircuit means for carrying out a recording method for opticallyrecording information, using an optical recording medium comprising aphase-change recording layer comprising Sb and Te as essential elementstherefor, to which is added at least one element selected from the groupconsisting of Ag, Au, Cu, Zn, B, Al, Ga, In, Si, Ge, Sn, Pb, N, P, Bi,La, Ce, Gd, and Tb, the recording layer being capable of assuming anamorphous phase changed from a crystalline phase by the application of alaser beam thereto, thereby optically recording information by formingrecording marks therein, wherein when the recording marks are formed inthe optical recording medium, a light emission wave of the laser beam isconverted into a recording pulse train comprising a plurality ofon-pulses and each off-pulse subsequent to the on-pulses, with arecording frequency ν (ν=1/Tw where Tw is a window width) beingcontinuously changed corresponding to the location of each of therecording marks in the radial direction of the recording medium, eitherin the direction from an inner circumference towards an outercircumference of the recording medium, or in the direction from theouter circumference towards the inner circumference of the recordingmedium, a plurality of on-pulses having a pulse width comprising (1) apulse width portion fixed with an identical time constant (T), and (2) apulse width portion determined by multiplying the window width (Tw) by aconstant, signal generation means for generating a signal correspondingto the time constant, and signal transmission means for transmitting thesignal to the driving circuit means.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view which shows the structure of aphase-change optical recording medium according to the presentinvention.

FIG. 2 is a chart in explanation of the recording method according tothe present invention.

FIG. 3 is a chart which shows the relationship between the recordinglinear velocity and the on-pulse width.

FIG. 4 is a chart which shows the relationship between the recordinglinear velocity in the recording method employed in Example 2 and thepulse duty ratio thereof.

FIG. 5 is a chart which shows the jitter performance in the recordingmethods employed in Examples 1 and 2.

FIG. 6 is a chart which shows the relationship between the recordinglinear velocity in the recording method employed in Example 3 and theon-pulse duty ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The optical recording medium of the present invention is characterizedin that the recording layer comprises Sb and Te as essential elementstherefor, to which is added at least one element selected from the groupconsisting of Ag, Au, Cu, Zn, B, Al, Ga, In, Si, Ge, Sn, Pb, N, P, Bi,La, Ce, Gd, and Tb, with the above-mentioned essential elements of Sband Te, and at least one element selected from the above-mentioned groupconstituting a eutectic phase-change material. It is preferable that atleast one element selected to serve as an additional component beingcontained in an atomic percentage of 17% or less in the eutecticphase-change material. As mentioned above, the recording layer for usein the present invention is different from the conventional onecomprising a compound substantially the same as Ge₂Sb₂Te₅₀.

FIG. 1 is a cross-sectional view which shows the structure of aphase-change optical recording medium according to the presentinvention. In an optical recording medium shown in FIG. 1, a firstdielectric layer 2 (serving as a lower protective layer), a recordinglayer 3, a second dielectric layer 4 (serving as an upper protectivelayer), and a metal reflection layer 5 (serving as a light reflectionand heat dissipation layer) are successively formed on a substrate 1which bears thereon a guide groove. Further, a protective layer 6comprising an UV curing resin may be preferably overlaid on the metalreflection layer 5.

In particular, it is preferable that the recording layer 3 comprise acomposition of AgInSbTe.

It is desirable that the first or second dielectric layer 2 or 4comprise a composition of ZnS—SiO₂.

It is also desirable that the metal reflection layer 5 comprise acomposition of Al—Ti.

As the material for the substrate 1, there can be generally employedglass, ceramic materials, and resins. A resin substrate is mostpreferable in light of the molding characteristics. Representativeexamples of the resin for the substrate are polycarbonate resin, acrylicresin, epoxy resin, polystyrene resin, polyethylene resin, polypropyleneresin, silicone resin, fluororesin, ABS resin, and urethane resin. Ofthese resins, the polycarbonate resin is preferably employed because ofthe advantages over other resins in terms of processability and opticalproperties. The substrate may be prepared in the form of a disc, card,or sheet.

Exemplary film-forming conditions of each layer in a film-formingchamber are shown below:

First Dielectric Layer (ZnS—SiO₂ Layer)

applied electric power: 3 kW

argon (Ar) gas pressure: 2 mmTorr (atmospheric pressure in afilm-forming chamber)

Recording Layer (AgInSbTe Layer)

applied electric power: 1 kW

argon (Ar) gas pressure: 2 mmTorr (atmospheric pressure in afilm-forming chamber)

Second Dielectric Layer (ZnS—SiO₂ Layer)

applied electric power: 3 kW

argon (Ar) gas pressure: 2 mmTorr (atmospheric pressure in afilm-forming chamber)

Metal Reflection Layer (Al Layer)

applied electric power: 9 kW

argon (Ar) gas pressure: 3 mmTorr (atmospheric pressure in afilm-forming chamber)

When the recording layer comprises a composition of AgInSbTe, thepresence of the element Ag can effectively improve the recordingcharacteristics and the preservation stability. However, the increase inan amount of Ag lowers the crystalline phase transition speed of therecording layer, so that it becomes difficult to cope with highrecording linear velocity. To cope with the rise of recording linearvelocity, the content of Sb or In may be increased. However, when thecontent of Sb is increased to cope with high recording linear velocity,there is a risk that the overwriting characteristics and thepreservation reliability under the circumstances of high temperature andhigh humidity will suddenly drop. When the content of In is increased,it is preferable that the atomic percentage of the element of In be 0.1or less.

The first dielectric layer and the second dielectric layer are formed bya variety of vapor growth methods, for example, vacuum deposition,sputtering, and electron beam evaporation. The thickness of the first orsecond dielectric layer varies depending on the function of the layer asa heat-resistant layer or multiple-interference layer. It is preferablethat the first dielectric layer have a thickness of 50 to 110 nm, or 170to 215 nm. When the thickness of the first dielectric layer is less than50 nm, the first dielectric layer cannot work to protect the substratefrom the influence caused by heat accumulation in the recording layer.When the thickness of the first dielectric layer is more than 215 nm,the peeling of the first dielectric layer from the interface can beprevented. It is also preferable that the thickness of the seconddielectric layer be in the range of 10 to 30 nm. The second dielectriclayer with such a thickness, the decrease of recording sensitivity andthe excessive heat accumulation can be prevented.

To erase the recorded information without fail in the course ofoverwriting operation, the conventional recording material needsdelicate control of temperature so that the recording layer is heated ataround the crystallization temperature thereof. In contrast to this, theerasing characteristics of the optical recording medium of the presentinvention are excellent. Further, consideration may be just given torapid cooling after the recording layer is irradiated with a laser beamin the course of recording. Therefore, a stable recording mark can beformed in the optical recording medium of the present invention by boththe CAV and CLV recording methods.

According to the present invention, recording marks are formed in theabove-mentioned optical recording medium by converting a light emissionwave of the laser beam into a recording pulse train comprising aplurality of on-pulses and each off-pulse subsequent to the on-pulses,with a recording frequency ν (ν=1/Tw where Tw is a window width) beingcontinuously changed corresponding to the location of each of therecording marks in the radial direction of the recording medium, eitherin the direction from an inner circumference towards an outercircumference of the recording medium, or in the direction from theouter circumference towards the inner circumference of the recordingmedium.

In the aforementioned recording method, there may be a difference inreflectance of 30% or more between (a) the crystalline phase and theamorphous phase formed by the application of the laser beam to thecrystalline phase at a recording linear velocity ranging from 3 to 20m/s.

FIG. 2 is a chart in explanation of one embodiment of theabove-mentioned recording method. In this case, the recording marks areformed in the recording medium in such a manner that a plurality ofon-pulses has a pulse width comprising a pulse width portion (dTmp)fixed with an identical time constant (T), and a pulse width portion(Tmp) determined by multiplying the window width (Tw) by a constant.

When the above-mentioned recording method as shown in FIG. 2, the pulsewidth of the on-pulses varies depending upon the recording linearvelocity, as indicated by a graph 1 shown in FIG. 3. In comparison withthe pulse width obtained by the above-mentioned recording methodaccording to the present invention, the pulse width obtained by theconventional recording method consists of only the pulse width portion(Tmp), so that the pulse width is constant regardless of the recordinglinear velocity, as indicated by a graph 2 in FIG. 3.

As is apparent from the graph 1 shown in FIG. 3, the above-mentionedrecording method of the present invention can increase the pulse widthat the lower linear velocity side, in other words, at the innercircumference of the recording medium. The result is that a stablerecording mark can be formed in the recording medium by any of the CAVor CLV recording system. Namely, it is possible to prevent thedeterioration of jitter performance which is conventionally caused insuch a way that the recording power becomes insufficient due to thenarrow pulse width at the inner circumference of the recording medium.

With respect to the pulse width portion (Tmp) in FIG. 2, it ispreferable that the ratio of the constant to the window width (Tw) is0.5 or less. When the aforementioned ratio is 0.5 or less, a stablepulse train for driving laser diode (LD) can be obtained. To be morespecific, the off-pulses become shorter as the recording linear velocitybecomes higher, in other words, as the window width becomes narrower. Insuch a case, a pulse decay time constant for laser diode (LD) cannot besecured, so that it becomes difficult to perform stable irradiation ofthe recording medium with the laser beam.

Furthermore, with respect to the pulse width portion (dTmp) fixed withan identical time constant (T), it is preferable that the ratio of thetime constant (T) to Tw be 0.8 or less in a maximum recording frequencyused, and 0.2 or more in a minimum recording frequency used. When suchthreshold values are provided, the pulse width can be inhibited frombecoming extremely narrow at the lower linear velocity side, so thatdeterioration of jitter performance due to insufficient recording powercan be effectively prevented.

In the case of the graph 1 in FIG. 3, both thresholds values aresatisfied. As a result, the duty ratio becomes larger at the lowerlinear velocity side, and therefore the jitter performance becomesbetter in the recording method indicated by the graph 1 than in theconventional recording method indicated by the graph 2. In FIG. 3, thisis expressed by the pulse width instead of the duty ratio forconvenience.

The recording method according to the present invention may employ incombination a recording mode (a) in which a plurality of on-pulses has apulse width comprising a pulse width portion (dTmp) fixed with anidentical time constant (T), and a pulse width portion (Tmp) determinedby multiplying the window width (Tw) by a constant, and a recording mode(b) in which a plurality of on-pulses has such a pulse width that isadjusted to have a constant duty ratio to the window width (Tw) in sucha manner that the recording mode (a) and the recording mode (b) areswitched at an intermediate recording frequency between a maximumrecording frequency and a minimum recording frequency.

A graph 1 in FIG. 4 shows the relationship between the duty ratio to thepulse width (Tw) at each linear velocity when the above-mentionedrecording method using the recording mode (a) and the recording mode (b)in combination is carried out.

When compared with the graph 1, according to a graph 2 of theconventional recording method, an off-pulse portion becomes shorter asthe window width decreases at the higher linear velocity. Therefore, apulse decay time constant of LD cannot be secured, so that stableirradiation of the recording medium with the laser beam cannot beachieved. At the lower linear velocity side, the pulse width becomes toonarrow to obtain sufficient recording power, so that the jitterperformance is lowered.

It is preferable that the recording mode (a) be carried out in a rangebetween the intermediate recording frequency and the minimum recordingfrequency. This case is indicated by a graph 1 in FIG. 6. It is possibleto prevent the deterioration of jitter values due to insufficientrecording power when the pulse width becomes too narrower at the lowerlinear velocity side. A graph 2 in FIG. 6 shows the relationship betweenthe linear velocity and the duty ratio when the conventional recordingmethod is employed.

To effectively prevent the above-mentioned drawbacks, it is preferablethat the constant duty ratio in the recording mode (b) be 0.8 or less.

To achieve the aforementioned recording methods, a recording apparatusof the present invention comprises laser beam driving circuit means,signal generation means for generating a signal corresponding to theaforementioned time constant, and signal transmission means fortransmitting the signal to the driving circuit means. Owing to such arecording apparatus, the pulse application time constant can be smoothlychanged. It is said that the recording apparatus particularly suitablefor CAV recording can be obtained.

Other features of this invention will become apparent in the course ofthe following description of exemplary embodiments, which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLE 1

Fabrication of Phase-Change Optical Recording Medium

On a polycarbonate substrate, a first dielectric layer (ZnS—SiO₂), arecording layer (AgInSbTe), a second dielectric layer (ZnS—SiO₂), and ametal reflection layer (Al—Ti) were successively overlaid by thesputtering method. An UV-curing resin was coated on the metal reflectionlayer by spin coating to provide a protective layer thereon.

Each of the above-mentioned layers was formed under the followingfilm-forming conditions:

First Dielectric Layer (ZnS—SiO₂ Layer)

applied electric power: 3 kW

argon (Ar) gas pressure: 2 mmTorr (atmospheric pressure in afilm-forming chamber)

Recording Layer (AgInSbTe Layer)

applied electric power: 1 kW

argon (Ar) gas pressure: 2 mmTorr (atmospheric pressure in afilm-forming chamber)

Second Dielectric Layer (ZnS—SiO₂ Layer)

applied electric power: 3 kW

argon (Ar) gas pressure: 2 mmTorr (atmospheric pressure in afilm-forming chamber)

Metal Reflection Layer (Al—Ti Layer)

applied electric power: 9 kW

argon (Ar) gas pressure: 2 mmTorr (atmospheric pressure in afilm-forming chamber)

Thus, a phase-change optical recording medium No. 1 according to thepresent invention was fabricated.

Using the optical recording medium No. 1, the CAV recording was carriedout in such a manner that the recording linear velocities were set to3.49 m/sec at the innermost circumference and 8.5 m/sec at the outermostcircumference. In this case, a plurality of on-pulses was controlled sothat a pulse width was composed of a pulse width portion fixed with atime constant of 8 nsec, and a pulse width portion determined bymultiplying the window width (Tw) by a constant of ⅙, as shown in FIG.2. A portion (dTera) shown in FIG. 2 was set by multiplying the windowwidth (Tw) by a constant of ⅙. In this case, the window width wasmultiplied by a positive constant, that is, +⅙. It was also possible tomultiply the window width by a negative constant, that is, −⅙. Further,the constant represented by 1/n where n is an integer is favorable inlight of the circuit.

The results of the recording operation are shown in FIG. 5. As isapparent from the graph in FIG. 5, the jitter value can be reduced atthe lower linear velocity side, thereby achieving stable recording.

For comparison, when recording was carried out by merely fixing the timeconstant, as shown in FIG. 5, the jitter value increased as a whole, inparticular, at the lower linear velocity side.

EXAMPLE 2

Using the same phase-change optical recording medium No. 1 fabricated inExample 1, the recording was carried out by combining the previouslymentioned recording mode (a) and recording mode (b). To be morespecific, the recording mode (a) was carried out in such a manner thatthe pulse width of a plurality of on-pulses was composed of a pulsewidth portion fixed with a time constant of 11.5 nsec, and a pulse widthportion determined by multiplying the window width (Tw) by a constant of⅙, as shown in FIG. 2. The recording mode (a) was adapted from theinnermost circumference of the recording medium up to a recordingposition of which the window width was 1.7 times that at the innermostcircumference. In other words, the recording mode (a) was switched tothe recording mode (b) at the position around half the distance of aradius of the recording medium. At this position, the recording linearvelocity was about 6 m/sec.

Outward from this position, the recording mode (b) was carried out withthe pulse width of a plurality of on-pulses being adjusted to have aconstant duty ratio to the window width of 0.6.

In this case, the duty ratio varied depending upon the recording linearvelocity as indicated by a graph 1 shown in FIG. 4. In contrast to thegraph 1, a graph 2 shows the relationship between the duty ratio and therecording linear velocity when the recording mode (a) employed inExample 2 was adapted up to the outermost circumference. In this case,as indicated by the graph 2, the duty ratio exceeded 0.8 at the positionof which the window width was 2.2 times that of the innermostcircumference, and the recording was impossible outerward therefrom.

The results of the recording operation are shown in FIG. 5 in terms ofthe jitter value depending upon the recording linear velocity.

EXAMPLE 3

Using the same phase-change optical recording medium No. 1 fabricated inExample 1, the recording was carried out by combining the previouslymentioned recording mode (a) and recording mode (b). To be morespecific, the recording mode (b) was first carried out with the pulsewidth of a plurality of on-pulses being adjusted to have a constant dutyratio to the window width of 0.5.

The recording mode (b) was adapted from the innermost circumference ofthe recording medium up to a position of which the window width was 1.7times that of the innermost circumference of the recording medium. Inother words, the recording mode (b) was switched to the recording mode(a) at the position around half the distance of a radius of therecording medium. At this position, the recording linear velocity wasabout 6 m/sec.

Outerward from this position, the recording mode (a) was carried out insuch a manner that the pulse width of a plurality of on-pulses wascomposed of a pulse width portion fixed with a time constant of 8 nsec,and a pulse width portion determined by multiplying the window width(Tw) by a constant of ⅙.

As a result, the jitter value was almost the same as that obtained inExample 2.

Japanese Patent Application No. 11-131926 filed May 12, 1999 is herebyincorporated by reference.

What is claimed is:
 1. A method for optically recording information,using an optical recording medium comprising a phase-change recordinglayer which is capable of assuming an amorphous phase changed from acrystalline phase by the irradiation of a laser beam thereto, therebyoptically recording information by forming recording marks therein,wherein when said recording marks are formed in said optical recordingmedium, said laser beam is modulated into a recording pulse traincomprising a plurality of high-power pulses and low-power pulsessubsequent to said respective high-power pulses, with a recordingfrequency ν (ν=1/Tw where Tw is a window width), and wherein saidrespective high-power pulses have a pulse width comprising (1) a pulsewidth portion fixed with a time constant (T), and (2) a pulse widthportion determined by multiplying said window width (Tw) by a constant.2. The recording method as claimed in claim 1, wherein there is adifference in reflectance of 30% or more between (a) said crystallinephase and said amorphous phase formed by the application of said laserbeam to said crystalline phase at a recording linear velocity rangingfrom 3 to 20 m/s.
 3. The recording method as claimed in claim 1, whereinin said pulse width portion fixed with a time constant (T), the ratio ofsaid time constant (T) to Tw is 0.8 or less in a maximum recordingfrequency used, and 0.2 or more in a minimum recording frequency used.4. The recording method as claimed in claim 1, wherein in said pulsewidth portion determined by multiplying said window width (Tw) by aconstant, the ratio of said constant to said window width is 0.5 orless.
 5. The recording method as claimed in claim 1, using incombination: (a) a recording mode in which said plurality of on-pulseshas a pulse width comprising (1) a pulse width portion fixed with anidentical time constant (T), and (2) a pulse width portion determined bymultiplying said window width (Tw) by a constant, and (b) a recordingmode in which said plurality of on-pulses has such a pulse width that isadjusted to have a constant duty ratio to said window width (Tw), insuch a manner that said recording mode (a) and said recording mode (b)are switched at an intermediate recording frequency between a maximumrecording frequency and a minimum recording frequency.
 6. The recordingmethod as claimed in claim 5, wherein said recording mode (a) is carriedout in a range between said intermediate recording frequency and saidminimum recording frequency.
 7. The recording method as claimed in claim5, wherein said constant duty ratio in said recording mode (b) is 0.8 orless.
 8. The recording method as claimed in claim 1, wherein therecording frequency changes continuously corresponding to the locationof each of the recording marks in the radial direction of the recordingmedium.